[RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available

[Christian König]

Am 02.07.24 um 11:06 schrieb Icenowy Zheng:
> [SNIP]
> However I don't think the definition of the AGP spec could apply on all
> PCI(e) implementations. The AGP spec itself don't apply on
> implementations that do not implement AGP (which is the most PCI(e)
> implementations today), and it's not in the reference list of the PCIe
> spec, so it does no help on this context.

No, exactly that is not correct.

See as I explained the No-Snoop extension to PCIe was created to help 
with AGP support and later merged into the base PCIe specification.

So the AGP spec is now part of the PCIe spec.
[SNIP]
>> We have quite a bunch of V4L, sound and I also think network devices
>> which work like that. But those are non-PCI devices.
> I think in the Linux kernel most drivers (of course including PCI ones)
> use DMA buffer in this way, which makes them natually compatible with
> non-coherent PCIe implementations. TTM is one of the few exceptions
> here.

Yes and that is absolutely intentional.

See we don't want to support any non-coherent PCIe implementation.

[SNIP]
>> And if I'm not completely mistaken the RISC-V specification was also
>> updated to disallow stuff like this.
>>
>> So yes you can have boards which implement non-snooped PCIe, but you
>> get
>> exactly zero support from hardware vendors to run software on it.
>>
> It's a quite usual case for free softwares to get no support from
> hardware vendors, and some of them are even developed by reverse
> engineering. I don't think it as a blocker for the Linux kernel to
> merge as many hardwares' support as possible.

We seem to have a misunderstanding here, this is not a software issue. 
The hardware platform is considered broken by the hardware vendor!

In other words people have stitched together hardware in a way which is 
not supported by the creator of that hardware.

So as long as you can't convince anybody from ARM or the RISC-V team or 
whoever created that hardware to confirm that the hardware actually 
works you won't get any support for that.

Regards,
Christian.
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